The present invention relates to a method of melting a cold material including iron and simultaneously obtaining high carbon and low phosphorous molten iron while keeping a high post combustion rate.
In Japanese Patent Unexamined Publication No. 60-174812 there is disclosed a converter steel-making method comprising: in the first step iron-containing cold material, carbonaceous materials and oxygen are supplied into a converter containing hot metal, called "hot heel", and high carbon molten iron is obtained in the second step the molten iron obtained in the first step is refined by oxygen blowing in another converter, and molten steel with a desired temperature and chemical composition is obtained.
The temperature of the above-mentioned molten iron in the first step is preferably 1450.degree. C. or lower for the purpose of minimizing a melting loss of a refractory material during the melting. In order to keep a heat source in the second step, the carbon content in the melt needs to be 3.0% or more, preferably 3.5% or more.
Another method of melting a cold material containing iron is disclosed in Japanese Patent Examined Publication No. 56-8085. This method for melting the cold material is effected by use of a converter 15 having, as shown in FIG. 1, an oxygen top-blowing lance 14 and a triple pipe nozzle 1 shown in FIGS. 1 and 2. In this method, there are fed iron-containing cold charge 17 such as scrap, sponge iron, pellet, solid pig iron and/or iron ore into the converter 15 in which a hot heel 16 such as pig iron is previously retained. Then, as shown in FIGS. 2 and 3, by use of a non-oxidizing gas such as nitrogen gas there is introduced through an inner pipe 2 of the triple pipe nozzle 1 a carbonaceous material such as coal powder or coke powder. Oxygen is simultaneously introduced into the converter through an intermediate pipe 3, while non-oxidizing gas such as LPG is also introduced through an outer pipe 4. As a result of the blowing, the carbonaceous material is dissolved in the bath so that a primary combustion (C+(1/2)O.sub.2 .fwdarw.CO) of the carbon in the bath occurs, and oxygen is further supplied through an oxygen top-blowing lance 14 so as to effect the post combustion (CO+(1/2)O.sub.2 .fwdarw.CO.sub.2) of CO. As a result of this, heat is supplied to the bath and the cold material is melted to obtain molten iron.
In FIGS. 2 and 3, reference numeral 7 represents projections provided on the outer periphery of the inner pipe 2 with the same interval while projections extend in the direction of the axis of the inner pipe. Each of the outer surfaces of the projections 7 is in contact with the inner periphery of the intermediate pipe 3 so as to form a gap 5. Reference numeral 8 represents projections provided on the outer periphery of the intermediate pipe 3 with the same interval which projections extends along an axis of the intermediate pipe. Each of the outer surfaces of the projections 8 is in contact with the inner periphery of the outer pipe 4 so as to form a gap 6. Reference numeral 9 represents a steel shell of the furnace body, and reference numeral 10 represents a refractory member provided on the inner periphery of the furnace body.
In this method of melting the iron-containing material, a post combustion is essential because the amount of raw materials such as carbonaceous material and oxygen used in the method of melting the iron-containing material such as steel scrap is, as shown in FIG. 4, determined by the rate of the post combustion. Therefore, the higher the post combustion rate is, the smaller the amount of the carbonaceous material and oxygen is when melting the iron-containing cold material.
According to the Japanese Patent Examined Publication No. 56-8085, a high post combustion rate can be obtained by a process having the steps of: disposing the oxygen top-blowing lance 14 so that the lance 14 is spaced apart 2 m or more from the surface of a bath; supplying oxygen of a free jet state from the level of 2 m or higher above the bath surface; and controlling the rate of bottom-blown oxygen in a range of 20 to 80% (the rate of the oxygen to be blown through the top-blowing lance 14 is 80 to 20%.) If the rate of the bottom-blown oxygen is less than 20%, there occurs the foaming of slag, which causes the space providing a free jet above the bath surface, to be reduced. As the result of foaming, a high post combustion rate cannot be obtained.
Although in the last-mentioned prior art method a, desulfuration reaction can be progressed in the converter during melting, a dephosphorization reaction, which is a oxidation reaction, cannot be progressed. Therefore, most of the phosphor contained in the raw materials is included in the molten iron. Therefore, a dephosphorization needs to be performed by use of slag for dephosphorization at the time of performing decarburization in a next decarburizing furnace.
On the other hand, the rate of bottom-blown oxygen is essential with respect to the both apparatus and operating cost. The lower the rate of the bottom-blown oxygen is, the simpler the bottom blowing equipment is (the number of the nozzles can be reduced), and the cost needed for the bottom-blowing equipments can be also reduced. Furthermore, the smaller the rate of the bottom-blown oxygen is, the smaller the amounts of non-oxidation cooling gas such as LPG and a protection gas such as N.sub.2 or Ar supplied for the purpose of preventing the clogging of oxygen-blowing nozzle is, so that an operation cost can be reduced.
On the other hand, since the bottom refractories of a furnace is apt to be readily damaged as the stirring force induceed to the bath becomes large, it is desired to reduce the rate of the bottom-blown oxygen so as to minimize the loss or damage of the refractories.
Japanese Patent Unexamined Publication No. 57-164908 discloses the same object as in the above-described Japanese Patent Examined Publication No. 56-8085, in which only the rate of bottom-blown oxygen is reduced to be not more than 20%, but other conditions are made to be similar to those of the Publication 56-8085. However, the mere reducing of the rate of the bottom-blown oxygen causes problems concerning the operation such as slag-foaming. In the method disclosed in the Japanese Patent Unexamined Publication No. 57-164908, no measure is taken to solve the problems, and a method of melting cold iron-containing material in which method the rate of oxygen blown from bottom is reduced to be not more than 20% is not realized. Furthermore, in the Publication No. 57-164908, there is no teaching for obtaining molten iron in which phosphor content is reduced, which reducing of phosphorus is one of the main object of the present invention.
Also in AIME annual meeting (March, 1987), there was disclosed a method of manufacturing steel comprising a first step in which iron-containing material is melted and a second step in which a high carbon molten iron prepared in the first step so as to be used as a raw material is refined by oxygen-blowing in another converter to thereby obtain molten steel of a desired temperature and composition. In this method, low phosphorous or low sulfur and high carbon molten iron is obtained in the first step so as to effect the second step without any dephosphorizing treatment or with a slight degree of dephosphorizing treatment to thereby obtain a merit of a slag-free refining. Regarding the conditions for achieving the above-object, the method discloses such matter that a slag containing a high rate of CaO is be provided under a condition that [C] is 3.5 to 4% at a temperature of 1400.degree. to 1450.degree. C. after the refining has been performed. Furthermore, the method discloses that it is preferable to make the temperature low at the time of refining. In addition, the conditions which enables the dephosphorization and desulfurization to be progressed sufficiently is considered in the method. According to the method, it is important to make slag having a melting point higher than the refining temperature, that is, it is important to make a solid state slag. In order to achieve this, the slag is made to have a composition in which the content of CaO is larger than the content of SiO.sub.2, that is, the slag is made to have a high basicity (CaO/SiO.sub.2). In order to obtain (P.sub.2 O.sub.5)/[P]=100 at the final stage of the method, the slag composition is made to have CaO/SiO.sub.2 .div.4.
In the operating conditions described regarding the prior art method, the following two serious problems arise.
First, if the slag is in a solid state after refining, removal of the slag from the melting furnace becomes impossible or very hard. If the slag is forcedly removed, the molten metal is also discharged with the result that the yield thereof is reduced. Furthermore, if the slag is in a solid state, molten metal is apt to adhere to or penetrate in the slag in a great degree, and the thus adhered metal is discharged with the slag. In this case, the yield is also reduced.
A second problem is that, a great quantity of CaO source needs to be used in order to make the slag having a high value of CaO/SiO.sub.2. That is, since a great quantity of SiO.sub.2 source exists in a melting furnace due to the carbonaceous materials and its scraps, a great quantity of the CaO commensurate to the former needs to be added.